The invention relates to a method for generating elemental bromine in electrolyte solutions used for operating metal-bromine cells, such as zinc-bromine batteries.
Zinc-bromine rechargeable cell contains two chemically non-reactive electrodes and a suitable separator located between the electrodes (e.g., an ion exchange membrane). The electrolyte used in the cell is an aqueous solution of zinc bromide, which is generally fed to the two compartments of the cell from two separate external reservoirs, utilizing a suitable circulation system. The term “anode” is used herein to indicate the electrode where metal zinc is formed (during charge) and oxidized (during discharge). The term “cathode” is used herein to indicate the electrode where elemental bromine evolves (during charge) and reduced (during discharge). The charge and discharge states of zinc-bromine battery will now be described in more detail.
During charge, an electric current is supplied to the cell from an external source, causing the deposition of zinc metal onto the anode and the concurrent generation of elemental bromine at the cathode, as shown by the following reaction:Zn2+(aq)+2Br−(aq)→Zn(s)+Br2(l) 
The aqueous electrolyte solution which circulates through the cathodic side during the cell charge contains a water soluble complexing agent which is capable of readily forming a water-immiscible liquid phase upon complexing with molecular bromine. Thus, the molecular bromine generated at the cathodic side during cell charge reacts almost instantaneously with the water-soluble complexing agent, to form a water immiscible oily phase. The dense bromine-containing oily phase tends to settle at the bottom of the reservoir used for holding the catholyte. The recirculation of the bromine-containing medium is prevented using suitable mechanical means, thus allowing the accumulation of elemental bromine in the catholyte reservoir. In this way, bromine is produced and stored in a reservoir outside the electrode.
During discharge, the reverse chemical reaction takes place and an electric current is drawn from the cell. The bromine-containing liquid, which forms part of the catholyte, is brought to the cathodic side of the cell, while the anolyte is simultaneously circulated through the anodic side. This results in the dissolution of the zinc anode to give zinc ions and the reduction of elemental bromine to form bromide ions (and the generation of electrical current). The chemical reaction is represented by the following equation:Zn(s)+Br2(l)→Zn2+(aq)+2Br−(aq) 
FIG. 1 provides a schematic illustration of an example of a zinc-bromine cell, wherein numerals 1a and 1c indicate the anode and cathode, respectively, and numeral 2 represents the separator positioned between the electrodes. A reservoir for accommodating an aqueous solution of zinc bromide, used as the anolyte, is indicated by numeral 3a. Similarly, a reservoir 3c contains the catholyte, which consists of two liquid phases: an upper, aqueous solution of zinc bromide and a lower, dense organic phase comprising the molecular bromine in a form of a complex. The flow paths allowing the circulation of the anolyte and catholyte are respectively indicated by arrows (the streams are driven by pumps Pa, Pc). A suitable valve (v) allows injection of bromine into the flow path of the catholyte on discharge only.
As explained in U.S. Pat. No. 5,702,842, on cell discharge, zinc fragments may detach from the surface of the electrode. The presence of these zinc fragments in the electrolyte may interfere with the efficient operation of the cell. For this reason, it is proposed in U.S. Pat. No. 5,702,842 to introduce, at the end of the discharge process, bromine-containing electrolyte into the electrode space where zinc is deposited, namely, at the anodic side, in order to chemically dissolve the undesired zinc fragments in the solution.
Accordingly, the introduction of a small amount of bromine to the anolyte, e.g., between about 0.1% and 1%, and more specifically between 0.3% and 0.7% by w/w (relative to the weight of the anolyte) at the end of the discharge process is considered to be beneficial. For example, a moderate capacity unit operating at 100 kW·h contains about one ton of an electrolyte solution, and therefore, a few kilograms of bromine are to be added to the anodic half-cell prior to charging. Similarly, for industrial units operating at 0.5-2 MW·h capacity, the initial amount of bromine required prior to starting a new unit charge cycle is up to 100 kg. However, molecular bromine is an easily volatile liquid with a strong, disagreeable odor an irritating effect. Therefore, the transportation and storage of molecular bromine must satisfy stringent requirements, and employing liquid bromine in populated areas requires the application of stringent safety measures and trained personal.